As the use of portable devices, such as high-performance notebook computers and mobile phones becomes increasingly popular in all over the world, a demand for high-performance secondary batteries with high energy density is being explosively increased. Particularly, lithium ion secondary batteries are being increasingly applied in most portable electronic products despite their short history, and thus, studies to extend the run-time of portable devices by increasing the capacity of lithium secondary batteries are being actively conducted. However, since an increase in the battery capacity leads to deterioration in the battery safety, there are various attempts to improve the safety of lithium secondary batteries.
Putting together the results of studies conducted up to now on the safety of the lithium secondary batteries, the thermal stability of active material and electrolyte in a charged state has the greatest effect on the battery safety. For this reason, studies to improve the safety of the lithium secondary batteries consist mainly of attempts to develop positive active materials with excellent thermal stability and studies to improve the thermal stability of electrolytes.
At present days, solvents used in electrolytes for the lithium secondary batteries are mainly cyclic and linear carbonates. Such solvents are inflammable and thus, upon either an increase in temperature caused by local short circuits within the batteries or an increase in surrounding temperature, the solvents will easily react with oxygen generated by the structural degradation of an active material, particularly a positive active material, resulting in combustion and explosion. For this reason, imparting flame retardancy to the electrolytes will greatly contribute to the improvement of the battery safety.
Many studies on flame retardant electrolytes which can be used in the lithium secondary batteries have been conducted centering around either solvents containing a compound having a fluorine-for-hydrogen substitution in carbonate or solvents containing phosphorus. Such a solvents have lower flammability and combustibility than those of the prior carbonate or ester solvents, but needs to be used at large amounts in order that electrolyte has sufficient flame retardancy. Also, such a flame retardant solvent shows a lower dissolution of lithium salts than that in the existing cyclic carbonates and contains fluorine or phosphorus with a higher atomic weight than that of hydrogen, resulting in a great increase in the viscosity of electrolytes. Thus, if the volume ratio of this flame retardant solvent in electrolyte solvent increases, the performance of batteries will be greatly deteriorated due to a great reduction in lithium ion conductivity.
Japanese Patent Laid-open Publication No. Hei 10-199567 discloses that if trifluoropropylene carbonate of the following formula 1 is used at the amount of 60-90% by volume relative to the total volume of electrolyte solvent, the safety of batteries can be improved:

However, trifluoropropylene carbonate has about two times higher viscosity than that of ethylene carbonate or propylene carbonate, a generally used solvent. Thus, if it is used at the amount described in the Japanese publication, it will result in a great reduction in the ion conductivity of electrolyte, thus making the deterioration of battery performance inevitable. Moreover, as the trifluoropropylene carbonate is a propylene carbonate substituted with fluorine, it has some of the disadvantages of propylene carbonate. Thus, if it is used in electrolyte solvent, the stability of a coating layer formed at the interface between a graphite negative electrode and an electrolyte will be somewhat insufficient and a problem in the charge/discharge life cycles of the battery will occur, as described in Electrochimica Acta Vol. 45, p. 99, 1999.
U.S. Pat. No. 6,506,524 describes a solvent consisting of fluoroethylene carbonate and propylene carbonate used as electrolyte solvent and the resulting electrolyte-stable protective layer capable of being formed on the surface of a graphite negative electrode material. However, if the solvent of this composition is used as electrolyte solvent, the ion conductivity of electrolyte will be reduced to less than 7 mS/cm, thus deteriorating the performance of batteries, because fluoroethylene carbonate and propylene carbonate have high dielectric constant but undesirably high viscosity.